Override samples and branched path tracing
The Render Layer 'Override' setting overrides the AA samples.

For example, setting the render layer override to 300 while having Diffuse branched path tracing samples set to 5 will result in 1500 Diffuse samples.

One interesting point to note is how the override interacts with the Square Samples setting as it overrides before the squaring takes place - so setting Override to, say, 300 with Square Samples enabled will be equivalent to setting to 90000 without squaring enabled.